The present invention generally pertains to a radiation detector and more particularly to an optical parallel-plate avalanche counter (“OPPAC”).
When traversing a material (gas, liquid or solid), a penetrating charged particle collides with the atoms or molecules of the medium and ionizes them to produce a trail of electron-ion pairs along its track. In conventional proportional gaseous counters (“PC”), upon the action of an electric field, charges are accelerated and drifted towards electrodes; positive ions move along the field direction while the electrons move opposite to it. When the electric field is strong enough, electrons reach high kinetic energies between collisions and eventually their energies exceed the ionization potential of gas molecules. This results in a further ionization that leads to an electron multiplication cascade known as a Townsend avalanche. The localization of the impinging particle in a position-sensitive proportional counter (“PSPC”) is determined from the amplitudes of signals on segmented/pixelated readout electrodes. For instance, the localization capability of a conventional two-dimensional Parallel Plate Avalanche Chamber (“2D-PPAC”) is based on recording the charge signals induced on two orthogonal striped readout foils connected to a resistive divider chain, on either side of a central biased electrode. The four signals at the ends of the two chains are amplified, shaped and the peak voltages recorded. The X and Y position is encoded in the ratio of the charges appearing at each end of the resistor chain (in a charge division method). Alternatively, the electrode strips may be connected to multi-tapped delay-lines, and the position is determined from the time difference between signals appearing at either end.
The localization capability of the PPAC is limited by the granularity of the readout foils; the latter usually consisting of stretched polymer (for example, polypropylene) films striped by evaporating a thin metal (for example, Au or Al) layer through a mask. Strips with a sub-millimeter gap and a center-to-center separation below of 1 mm are difficult to realize, so that best position resolutions are of 1 mm or above, depending on the charge readout methods, the geometry of the strip readout, and the identity of the impinging particle. The main limitation of PPAC is their tendency to discharge at high-gain operation; due to the large energy stored in the detector, a spark can damage both the detector and the readout electronics. The counting rate capability of conventional PPACs with a charge-division readout method is limited to a few tens of KHz, while the delay-line PPAC is of a few hundred of KHz. However, delay-line PPAC have generally lower detection efficiency due to a worse signal-to-noise ratio compared to charge-division PPAC.
Examples of conventional PPAC detectors are discussed in: Mantovan, R. et al., “Development of a Parallel-Plate Avalanche Counter to Perform Conversion Electron Mössbauer Spectroscopy at Low Temperatures,” Rev. Sci. Instrum. 78, 063902 (Jun. 6, 2007); Cub, J. et al., “A Position Sensitive Parallel Plate Avalanche Counter for Single Particle and Current Readout,” Nucl. Instr. and Meth. A 453 (2000) 522-524; and Swan, D. et al., “A Simple Two-Dimensional PPAC,” Nucl. Instr. and Meth. A 348 (1994) 314-317.